1. Field of the Invention
The present invention relates to a multibeam scanner.
2. Description of Related Art
Beam scanners used in laser beam printers and the like use a polygon mirror or the like to deflect an optical beam that is modulated according to image signals, thereby scanning a photosensitive material in a main scanning direction, while moving the photosensitive material in an auxiliary scanning direction to expose the photosensitive material to form an image thereon.
In recent years, a multibeam scanner has been proposed for performing main scans over a plurality of scanning lines simultaneously using a plurality of laser beams to irradiate the photosensitive material. By scanning a plurality of scanning lines in the main scanning direction simultaneously, this multibeam scanner can expose a photosensitive material in a shorter time than scanners using only one laser beam. Further, since the laser beams that scan the photosensitive material simultaneously are maintained at fixed intervals or distances in relation to one another, irregular intervals between beams is less likely to occur.
The scanning velocity in the auxiliary scanning direction can deviate. When the interval between scanning lines becomes narrower, the image density or print density increases. When the interval becomes wider, the image density decreases.
Japanese Patent No. 2575849 has proposed an image exposing apparatus which combines three different color beams together into a single light beam and then scans the single light beam onto a recording material. The recording material is scanned simultaneously by the single light beam only. According to this publication, an encoder is provided to detect an irregular conveying speed of the recording material in the auxiliary scanning direction. When the encoder detects such irregularity in the conveying speed, the overall exposure amount is regulated in order to correct irregularities in image density that is caused by the irregular conveying speed. That is, the exposure amounts of light beams in all the colors are adjusted dependently on the results detected by the encoder.
When speed variations in the auxiliary scanning direction occur in multibeam scanners, however, some intervals are varied by the change in velocity, but some other intervals between scanning lines are fixed. Therefore, changing the exposure amount in all of the laser beams will loss the exposure balance, preventing the formation of a detailed image with a balanced image density.
In view of the above-described drawbacks, it is an object of the present invention to provide an improved multibeam scanner that is capable of exposing a detailed image at a high rate of speed and without density irregularities by appropriately adjusting the exposure amount in the scanning lines.
In order to attain the above and other objects, the present invention provides a multibeam scanning device, comprising: a photosensitive medium moving unit moving a photosensitive medium in a predetermined moving direction, an auxiliary scanning direction being defined as a direction opposite to the predetermined moving direction; a light beam unit that emits a plurality of light beams; a beam deflecting unit deflecting the plurality of light beams, which are modulated according to image signals, to simultaneously scan the plurality of light beams on a corresponding plurality of scanning lines that extend along a predetermined main scanning direction on the photosensitive medium, while the photosensitive medium moves in the predetermined moving direction, thereby exposing a corresponding image on the photosensitive medium, the main scanning direction being substantially orthogonal to the moving direction, the plurality of scanning lines being arranged adjacent to one another in the auxiliary scanning direction; a movement velocity detector detecting the movement velocity of the photosensitive medium along the predetermined moving direction; and a light intensity control unit controlling the light beam unit to adjust the intensity of a part of the plurality of light beams based on the detected moving velocity of the photosensitive medium.
The light intensity control unit may control the light beam unit to adjust, based on the detected moving velocity of the photosensitive medium, the intensity of at least one of two light beams in the plurality of light beams, the two light beams including a first light beam and a second light beam, the first light beam being scanned in a first scanning line that is located in a most upstream side of the plurality of scanning lines along the auxiliary scanning direction, the second light beam being scanned in a second scanning line that is located in a most downstream side of the plurality of scanning lines along the auxiliary scanning direction.
The beam deflecting unit may deflect the plurality of light beams in the main scanning direction repeatedly to form a successive sets of plural scanning lines while the photosensitive medium moving unit moves the photosensitive medium in the predetermined moving direction, thereby forming the successive sets of scanning lines to be arranged in the auxiliary scanning direction. The light intensity control unit may control the light beam unit to adjust, based on the detected moving velocity of the photosensitive medium, the intensity of at least one of the first and second light beams, the first light beam forming, in each set, a first scanning line that is located adjacent to a scanning line in a preceding set of scanning lines, the second light beam forming, in each set, a second scanning line is that located adjacent to a scanning line in a subsequent set of scanning lines.
The light intensity control unit may include: a velocity comparing unit comparing the detected velocity with a predetermined reference velocity; and an intensity changing unit that changes the intensity of at least one of the first and second light beams based on the compared result. The light beam unit initially may emit each of the plurality of light beams at a predetermined intensity. The intensity changing unit may change the intensity of the at least one light beam from the predetermined intensity when the compared result shows that the moving velocity of the photosensitive medium is different from the predetermined reference velocity. For example, the intensity changing unit may increase the intensity of the at least one light beam from the predetermined intensity when the compared result shows that the moving velocity of the photosensitive medium is greater than the predetermined reference velocity. The intensity changing unit may decrease the intensity of the at least one light beam from the predetermined intensity when the compared result shows that the moving velocity of the photosensitive medium is smaller than the predetermined reference velocity.